The present invention relates to a semiconductor laser device including a semiconductor laser chip.
A semiconductor laser device provides an optical output by operating a semiconductor laser chip with the application of a forward bias thereto. However, if a surge voltage has been applied to the semiconductor laser chip, then an excessive amount of surge current flows through the semiconductor laser chip in the forward direction, resulting in an excessive optical output. Consequently, the performance of the semiconductor laser chip itself deteriorates.
First, a conventional surge-suppressive semiconductor laser device will be described with reference to FIGS. 8(a) and 8(b).
FIG. 8(a) is perspective view of a conventional semiconductor laser device. As shown in FIG. 8(a), a semiconductor laser chip 23, made of GaAlAs, is formed over a ceramic capacitor 21 with a laser mount 22 interposed therebetween. The ceramic capacitor 21 is connected in parallel to the semiconductor laser chip 23 via a wire 24.
FIG. 8(b) is circuit diagram of the conventional semiconductor laser device. As shown in FIG. 8(b), the semi-conductor laser chip 23 and the ceramic capacitor 21 are connected in parallel to each other. When a surge voltage is applied to the semiconductor laser device, the ceramic capacitor 21 absorbs a surge current, which is a transient current, thus preventing an excessive amount of current from being supplied to the semiconductor laser chip 23. Also, if the impedance of the ceramic capacitor 21 is made sufficiently smaller than that of the semiconductor laser chip 23 by increasing the capacitance of the ceramic capacitor 21, then the surge current can be absorbed even more effectively.
In the conventional semiconductor laser device, in order to absorb the surge current sufficiently, the impedance of the ceramic capacitor 21 should be smaller than that of the semiconductor laser chip 23. Thus, the capacitance of the ceramic capacitor 21 should be set larger than the junction capacitance of the semiconductor laser chip 23.
However, suppose a reference voltage, applied to a laser driver for driving the semiconductor laser chip 23, fluctuates; the semiconductor laser chip 23 is superimposed with a radio frequency voltage; or a pulse voltage is applied to the semiconductor laser chip 23 when the capacitance of the ceramic capacitor 21 is larger than the junction capacitance of the semiconductor laser chip 23. In each of these situations, radio frequency components leak to the ceramic capacitor 21 connected in parallel to the semiconductor laser chip 23. As a result, the response characteristics of the semiconductor laser chip 23 adversely deteriorate.